The present invention relates to sun sensors and in particular to sun sensors that provide the angular location of the sun with respect to the sensor reference frame.
Sun sensors are required equipment for most satellites since they provide line-of-sight direction of the sun with respect to the satellite. Such sensors are useful during several stages of the satellite mission including transfer orbit; pre- and post-apogee motor firing; attitude acquisition; momentum dump attitude sensing; loss of attitude lock and re-acquisition; and other emergency and routine requirements. Sun sensors of various types are also used for solar array tracking, thermal control and sun presence detectors for star sensors and earth sensors. Sun sensors are also useful for ground-based equipment that require the location of the sun to be known.
The sun Is a very bright source emitting throughout the visible and infrared regions corresponding to a black body with a temperature of about 5900 K This means that it has a peak in its spectral radiance at about 500 nm in the middle of the visible band. Thus most instruments that view the sun use the visible region. Inevitably, much of the solar power falling onto such an instrument must be discarded to avoid overloading or overheating the detectors and other portions of the instrument optics. However, the sun is so intense that stray light issues can arise that remain significant even if rejection is high.
Given that the satellite is in orbit around the Earth, the location of the sun changes continually with respect to the satellite body axes. For equatorial orbits, the sun is constrained in elevation by the ecliptics. However, for other orbits this is not the case and a broader field-of-view, FOV, is required to continually monitor the sun""s location. For the example case of a geosynchronous telecoms satellite and considering an Earth-centred set of coordinate axes, the operational range of the sun will be xc2x123.5 degrees in elevation from the orbit plane. During the day, the sun will appear to move through approximately 360 degrees in the azimuth plane. Normal and emergency operations as indicated above will require occasional access to a broader elevation range.
The accuracy required for a sun sensor is governed by its use in the attitude control strategy of the satellite mission. For the telecommunication satellite missions, the accuracy requirement is usually derived from antenna pointing specifications which are in turn driven by gain slopes attainable for the various radio frequency beams. As technology advances and tighter beams are utilized for point-to-point communications, the gain slopes are increasing requiring tighter tolerances on attitude control. Thus attitude resolution of an arc-minute or less is desirable and in certain instances may be necessary.
The combined problems of wide angular FOV and high resolution represent the major challenge to sensor designers. Most instruments must compromise between these two contradictory requirements. Having a FOV of 120xc3x97120 degrees with a resolution of 0.02 degree implies 6000 resolution elements in each direction. Thus multiple sensors are often used to provide a wide field-of-regard, FOR, (multiple FOV""s) while maintaining the necessary resolution.
The sun subtends an angular size of 0.53 degree. For most instruments the sun can be considered a point source. As higher resolutions (such as 0.02 degree) are demanded this assumption must be re-examined and its implications considered,
Manufacturers of digital sun sensors have relied on analog technology for the sensing elements but superimpose elaborate masks to provide the necessary resolution, Thus the sun at a given angle illuminates certain sensing photocells through a main slit and a mask consisting of reticle slits. The digital signal is usually produced as Gray code information based on which photocells produce voltages above a threshold level. This technique is only effective in a single axis so that a two-axis sensor consists of a pair of sensors mounted orthogonally.
This technology has limitations in regard to resolution owing to the sensitivity of the photocells and the accuracy and alignment of the mask slits. The sensors and the associated electronics are usually separate units. A typical sensor head with an FOV of 128xc3x97128 degrees and a resolution of 0.25 degree has a volume of 130 cm3 and a mass of 260 g whereas the associated electronics has a volume of 315 cm3 and a mass of 295 g. Thus a system for full sky viewing consisting of five sensor heads and one electronics box has a total volume requirement of 965 cm3 and a total mass of 1595 g. The power requirement for the system is about 120 mW. Higher accuracy units require significantly more resources especially for the mass, volume, and power for the processing electronics unit.
Thus it would be advantageous to prove a sun sensor that has a smaller volume and a smaller mass.
The present invention provides an implementation of a sun sensor that provides high resolution over a wide FOR. The sensor uses the principles of a classic pinhole camera in conjunction with a modem two-dimensional detector array. The sensor uses multiple pinholes located in a dome-like housing over the detector array to provide multiple FOV""s that project onto a common array. The FOV""s are positioned such that when taken in combination comprise the required wide FOR. The overlay of multiple sky images provided by the multiple pinholes onto the common detector allows for the monitoring of the entire FOR since the sun is a unique target within a background that is generally undifferentiated. Thus the sensor takes advantage of this xe2x80x9coverlay principlexe2x80x9d to multiplex the many FOV""s required to retain a high resolution and a wide FOR simultaneously.
A position or sun sensor comprises a sensor housing, a plurality of pinholes formed in the sensor housing, a detector mounted within the housing and a method of processing the information detected. The detector is mounted in the sensor housing. Each pinhole has a field of view and the detector receives the images from each field of view. Each field of view is defined by the position of the pinhole relative to the detector. The images are received in an overlay relationship thereby providing a field of regard. The processing method determines the presence and location of an object in a field of regard.
Further features of the invention will be described or will become apparent in the course of the following detailed description.